1. Field of the Invention
This invention relates to a laser marker employing a broad area semiconductor laser emitting multimode laser light which is focused by an optical system, thereby marking an object made of a metal or resin, for example, and a method of light spot adjustment for the laser marker.
2. Description of the Prior Art
A broad area semiconductor laser is conventionally used as an oscillator for laser markers. This type of semiconductor laser has an active region having a large area so that a high output is obtained. However, a stripe of the active region has a larger width than normal semiconductor lasers. Accordingly, laser light emitted from the broad area semiconductor laser has the shape of an ellipse which is long perpendicularly to a plane of an active layer. An ideal circular small light spot cannot be obtained even when the aforesaid laser light is focused by a usual optical system. As a result, an obtained energy density is not sufficient to mark an object. To overcome this drawback, the prior art has used a special optical system for reshaping the small light spot into a substantially circular shape.
FIGS. 11 and 12 show one of the aforesaid special optical systems. As shown, laser light emitted from a semiconductor laser 1 is converted by a collimator lens 2 to parallel beams of light. The light beams are further caused to pass through an anamorphic prism 5 so that a width of the light parallel to the plane of the active layer is increased so as to become approximately equal to a width of the light perpendicular to the plane of the active layer. By application of the above-described optical system, a generally circular laser light is caused to impinge onto the focusing lens and focused so that a circular small light spot is obtained.
However, the above-described constitution has the following problems. First, the width of the parallel light needs to be increased with increase in a stripe width of the active region of the semiconductor laser 1. This increases geometrical dimensions of the anamorphic prism 3 and accordingly increases the size of the optical system, resulting in increases in the weight and cost of the system.
Secondly, the broad area type semiconductor laser 1 has a large astigmatism. More specifically, the aforesaid perpendicular laser light and the parallel laser light have apparent outgoing positions (the centers of divergent pencil of rays) shifted from each other in the direction of an optical axis to a large extent. Accordingly, when the location of the collimator lens is adjusted so that the perpendicular laser light becomes parallel light beams, the parallel laser light having passed through the collimator lens 2 does not become parallel light beams by an adverse effect of the astigmatism. Thus, the lens 4 focuses the perpendicular laser light but does not focus the parallel laser light, whereupon a small light spot cannot be obtained. In order that this problem may be solved, a cylindrical lens 5 with a long focal distance is provided in the rear of the collimator lens 2 to correct the astigmatism so that the parallel laser light becomes parallel light beams.
However, since the astigmatism varies from one semiconductor laser to another, the cylindrical lenses 5 need to have different focal distances according to the variations in the astigmatism. This requires various types of cylindrical lens, resulting in an increase in the manufacturing cost.
Thirdly, the focal distance of the focusing lens 4 generally has an error ranging between 2 and 3%. Accordingly, the location of the focusing lens 4 needs to be adjusted according to the error of the focal distance. This adjustment is troublesome.
Therefore, an object of the present invention is to provide a laser marker in which the laser light emitted from the broad area type semiconductor laser can be focused into a small light spot by the optical system without an increase in the size thereof.
The present invention provides a laser marker comprising a collimator lens converting laser light emitted from a broad area semiconductor laser into substantially parallel beams of light, a cylindrical concave lens permitting to pass therethrough the laser light having passed through the collimator lens and being perpendicular to a plane of an active layer of the semiconductor laser, the cylindrical concave lens serving as a concave lens for the laser light having passed through the collimator lens and being parallel to the plane of the active layer, a cylindrical convex lens permitting to pass therethrough the laser light having passed through the concave lens and being perpendicular to the plane of the active layer of the semiconductor laser, the cylindrical convex lens serving as a convex lens for the laser light having passed through the collimator lens and being parallel to the plane of the active layer, and a focusing lens focusing the laser light having passed through the cylindrical convex lens.
According to the above-described construction, the laser light emitted from the semiconductor laser is converted into the substantially parallel light beams when passing through the collimator lens. The parallel light beams impinge through the cylindrical concave and convex lenses onto the focusing lens. The focusing lens focuses the incident perpendicular laser light and parallel laser light.
The location of the collimator lens is adjusted so that an incident angle of the perpendicular laser beam incident onto the focusing lens can be adjusted. Further, the locations of the cylindrical concave and convex lenses are adjusted so that an incident angle of the parallel laser beam incident onto the focusing lens can be adjusted. Accordingly, the laser light can be focused by the focusing lens into the small light spot. In this case, by the adjustment of the optical system, the incident angles of the perpendicular laser light and the parallel laser light both incident onto the focusing lens are adjusted so that a focusing location of the small light spot is adjusted.
Even when the apparent outgoing locations of the perpendicular laser light and the parallel laser light are shifted from each other by the influence of astigmatism, the influence can be prevented since the locations where the perpendicular laser light and the parallel laser light are focused respectively can be adjusted independent of each other. Further, the incidence angles of the perpendicular laser light and the parallel laser light both incident on the focusing lens can be adjusted. Consequently, the influences of the differences in the focal distances of the focusing lenses can be canceled and the laser light can be focused at a target location by the focusing lens.
In a preferred form, the collimator lens and the cylindrical concave lens are movable together. Consequently, optical axes of the respective lenses can be caused to accurately agree with each other. Further, the incidence angle of the perpendicular laser light incident on the focusing lens can be adjusted. In this case, the incidence angle of the parallel laser light incident on the focusing lens is varied as the result of simultaneous movement of the cylindrical concave lens. However, the location of the cylindrical convex lens can be adjusted so that the incidence angles of the respective laser beams perpendicular and parallel to the active layer of the semiconductor laser are equal to each other when the perpendicular laser light and the parallel laser light are equidistant from an optical axis. Additionally, the incidence angle of the perpendicular laser light incident on the focusing lens is not varied even when the location of the cylindrical convex lens is adjusted as described above. Consequently, the laser light can be focused by the focusing lens into a small light spot.
In another preferred form, the focusing lens is fixed. The incidence angles of the perpendicular laser light and the parallel laser light incident on the focusing lens can be adjusted so that the location where the laser light is focused by the focusing lens can be adjusted.